1. Field of the Invention
The present invention relates to a magnetoresistance effect element.
2. Description of the Related Art
In general, information recorded on a magnetic recording medium has been read out by detecting a voltage induced in a coil by electromagnetic induction. The induction occurs when a reproducing magnetic head having the coil moves in relation to the recording medium. On the other hand, it has been known to use a magnetoresistance effect element (hereinafter referred to as an MR element) in case of reading out information (refer to IEEE MAG-7, 150 (1971) and the like). A magnetic head using an MR element (hereinafter referred to as an MR head) utilizes a phenomenon that electric resistance of a ferromagnetic material of some kind varies according to intensity of an external magnetic field.
In recent years, since making a magnetic recording medium smaller in size and greater in capacity has been promoted and a relative velocity between a reproducing magnetic head and a magnetic recording medium when reading out information has been smaller, the expectation on an MR head which can have a great output for a small relative velocity has been increased.
As described above, a magnetoresistance effect is a phenomenon that electric resistance of a ferromagnetic material is changed by an external magnetic field, and its application has been studied as a high-sensitivity magnetic sensor (for example a head for magnetically recording and reproducing).
Although ferromagnetic material by itself has an anisotropic magnetoresistance effect (AMR), it only shows a resistance change ratio of at most only a few percentages. On the other hand, a giant magnetoresistance effect (GMR) depends on the directions of magnetization of ferromagnetic conductive layers superposed with a non-magnetic conductor layer interposed between the ferrogagnetic layers. This type of structure can give a resistance change ratio exceeding 10%, making it a promising high-sensitivity magnetic sensor, particularly as a read head for magnetic recording.
Magnetoresistance effect elements of the GMR type may comprise an element having a magnetic exchange coupling between ferromagnetic layers (for example a metallic magnetic artificial lattice element) of an element having substantially no exchange interaction (for example a spin valve element).
In order to detect a weak magnetic field in a read head or the like, it is necessary to obtain a great resistance change with a weak magnetic field. Therefore, both a great resistance change ratio and a little saturation magnetic field (a magnetic field where resistance change caused by an applied magnetic field is saturated) are required. Accordingly, in a magnetoresistance effect element of a GMR type, it is required for a ferromagnetic layer forming a laminated film to have a soft magnetic property.
In order to attain such a soft magnetic property, it will do to use a soft magnetic material. However, some materials may be low in resistance change ratio, cannot bear various conditions in manufacturing the element, or may have a problem in durability at the time of operation of the element.
For example, an NiFe alloy (Permalloy or the like), which has a little coercive force, is a typical soft magnetic material, but it is low in Curie temperature and poor in thermal resistance in comparison with a Co alloy. On the other hand, a Co alloy has a great coercive force and has a problem in a soft magnetic property, and its saturation magnetic field is great. Thus, it has been difficult to satisfy the desired properties by selecting materials for the ferromagnetic conductor layers.
An NiFe alloy (a Permalloy alloy) has been used in a part varying in resistance by sensing an external magnetic field outside an MR head (which part is hereinafter referred to as an MR element). However, even a Permalloy alloy having a good soft magnetic property is at most 3% in magnetoresistance change ratio, and is insufficient in magnetoresistance change ratio as an MR element to be used for a magnetic recording medium which has been made small in size and great in capacity. Therefore, an MR element material showing a more high-sensitivity magnetoresistance effect has been desired.
In order to meet such a demand, it has been confirmed that a high magnetoresistance effect occurs in a multi-layered film where ferromagnetic metallic films and non-magnetic metallic films are alternately laminated under a certain condition and where the ferromagnetic metallic films near each other are coupled by antiferromagnetic coupling. Such a structure is called an artificial lattice film. It has been reported that an artificial lattice film shows a great magnetoresistance change ratio of more than 100% at the maximum (see "Phys. Rev. Lett., Vol. 61,2474 (1988), "Phys. Rev. Lett., Vol. 64,2304 (1990) and the like). However, since an artificial lattice film has a high saturation magnetic field, it is generally unsuitable for an MR element.
On the other hand, an example has been reported that a multilayered film of a sandwich structure of a ferromagnetic film/a non-magnetic film/a ferromagnetic film, in which the ferromagnetic films are not coupled by antiferromagnetic coupling, also has a great magnetoresistance effect. Namely, one of the two ferromagnetic films having a non-magnetic film interposed between them is fixedly magnetized by applying an exchange bias to it. The other ferromagnetic film is reversed in magnetization by an external magnetic field (a signal magnetic field or the like). In this way, a great magnetoresistance effect can be obtained by changing a relative angle made between the directions of magnetization of the two ferromagnetic films disposed so as to have a non-magnetic film interposed between them. A multilayered film of such a type is called a spin valve film (see "Phys. Rev. B., Vol.45,806 (1992), "J. Appl. Phys., Vol. 69,4774 (1991), and the like). Although a spin valve film has a smaller magnetoresistance change ratio in comparison with an artificial lattice film, it is suitable for an MR element because it can saturate magnetization in a low magnetic field. A great expectation in practical use is placed on an MR head using such a spin valve film.
In an MR element using a spin valve film where the ferromagnetic films are not coupled by antiferromagnetic coupling, element sensitivity, can be improved by improving a soft magnetic property of the ferromagnetic film to be reversed in magnetization by an external magnetic field. However, since a material bringing a great MR change quantity does not always show a good soft magnetic property, it has been an important problem to reconcile these two properties with each other.
For example, although a spin valve film using a Co film or a Co based magnetic alloy film as a ferromagnetic film, or a crystalline Co film or a crystalline Co based magnetic alloy film, shows a good MR change quantity, it is difficult to achieve a soft magnetic property. In general, in Co or a Co based magnetic alloy, dispersion of magnetic anisotropy is generated. In addition, a great coercive force H.sub.c is generated in magnetic hysteresis in a direction of hard axis of magnetization. Therefore, a sensor device using an MR element having a spin valve film using Co or a Co based magnetic alloy, the deterioration of the S/N ratio caused by Barkhausen noises will make use of the sensor device impratical.
Particularly, in a spin valve film, it is desirable to make the directions of magnetization of the two ferromagnetic films having a non-magnetic film interposed between them nearly perpendicular to each other in the zero magnetic field. This is done through an annealing process. However, when performing such an annealing process, a problem happens that dispersion of magnetic anisotropy becomes greater and the S/N ratio is deteriorated by Barkhausen noises.
Thus, in an MR element using a spin valve film, it has been a problem to realize a good soft magnetic property in good repeatability in a spin valve film having a great MR change quantity and a low coercive force H.sub.c in a direction of hard axis of magnetization by suppressing dispersion of magnetic anisotropy. It is also desired to suppress Barkhausen noises in case of using an MR element using a spin valve film as a sensor device by reducing a coercive force H.sub.c in direction of hard axis of magnetization.